Date:	Tue, 24 Apr 2012 17:30:46 +0200
From:	Jan Kara <jack@...e.cz>
To:	Jeff Liu <jeff.liu@...cle.com>
Cc:	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	linux-ext4@...r.kernel.org, Jan Kara <jack@...e.cz>, tytso@....edu
Subject: Re: [PATCH] quota: fix a potential dead lock at add_dquot_ref()
 when performing quotaon against ext4 with files was writing

  Hello,

On Tue 24-04-12 14:25:44, Jeff Liu wrote:
> I just ran into an issue on vanilla-kernel-3.3.0 when executing quotaon(8) on ext4 with "usrquota,grpquota", and there
> have files are being writing at the same time.
> 
> I have a lxc guest running on an ext4 partition, 
> $ mount|grep sda6
> /dev/sda6 on /ext4 type ext4 (rw,usrquota,grpquota)
> 
> Execute quotacheck -cvumg /ext4 to force the quota checking without shutdown that guest, at this stage, everything is fine,
> # quotacheck -cvgum /ext4
> quotacheck: Your kernel probably supports journaled quota but you are not using it. Consider switching to journaled quota to avoid running quotacheck after an unclean shutdown.
> quotacheck: Scanning /dev/sda6 [/ext4] done
> quotacheck: Cannot stat old user quota file /ext4/aquota.user: No such file or directory. Usage will not be subtracted.
> quotacheck: Cannot stat old group quota file /ext4/aquota.group: No such file or directory. Usage will not be subtracted.
> quotacheck: Cannot stat old user quota file /ext4/aquota.user: No such file or directory. Usage will not be subtracted.
> quotacheck: Cannot stat old group quota file /ext4/aquota.group: No such file or directory. Usage will not be subtracted.
> quotacheck: Checked 3357 directories and 39335 files
> quotacheck: Old file not found.
> quotacheck: Old file not found.
> 
> However, the kernel was hang when running quotaon /ext4.
> 
> I observed the following info via netconsole:
> 
> [  423.140177]  *** DEADLOCK ***
> [  423.140177] 
> [  423.140177]  May be due to missing lock nesting notation
> [  423.140177] 
> [  423.140177] 4 locks held by quotaon/2350:
> [  423.140177]  #0:  (&type->s_umount_key#26){++++++}, at: [<c122248e>] get_super+0xf9/0x1ec
> [  423.140177]  #1:  (&s->s_dquot.dqonoff_mutex){+.+...}, at: [<c129b736>] vfs_load_quota_inode+0x3e5/0xac6
> [  423.140177]  #2:  (inode_sb_list_lock){+.+...}, at: [<c129b99b>] vfs_load_quota_inode+0x64a/0xac6
> [  423.140177]  #3:  (&sb->s_type->i_lock_key#16){+.+...}, at: [<c129b9de>] vfs_load_quota_inode+0x68d/0xac6
> [  423.140177] 
> [  423.140177] stack backtrace:
> [  423.140177] Pid: 2350, comm: quotaon Not tainted 3.3.0 #79
> [  423.140177] Call Trace:
> [  423.140177]  [<c182b385>] ? printk+0x57/0x6a
> [  423.140177]  [<c10ee73e>] __lock_acquire+0x133d/0x1a8a
> [  423.140177]  [<c105da85>] ? vprintk+0x910/0x93a
> [  423.140177]  [<c1298068>] ? inode_get_rsv_space+0x45/0x8b
> [  423.140177]  [<c10ef795>] lock_acquire+0x13a/0x176
> [  423.140177]  [<c1298068>] ? inode_get_rsv_space+0x45/0x8b
> [  423.140177]  [<c182f778>] _raw_spin_lock+0x54/0x7d
> [  423.140177]  [<c1298068>] ? inode_get_rsv_space+0x45/0x8b
> [  423.140177]  [<c1298068>] inode_get_rsv_space+0x45/0x8b
> [  423.140177]  [<c129bbe2>] vfs_load_quota_inode+0x891/0xac6
> [  423.140177]  [<c129c1cb>] dquot_quota_on+0x82/0x97
> [  423.140177]  [<f825d352>] ext4_quota_on+0x191/0x219 [ext4]
> [  423.140177]  [<c106e4e5>] ? ns_capable+0x71/0xa3
> [  423.140177]  [<c129e300>] do_quotactl+0x2f7/0x80f
> [  423.140177]  [<f825d1c1>] ? ext4_msg+0x61/0x61 [ext4]
> [  423.140177]  [<c122248e>] ? get_super+0xf9/0x1ec
> [  423.140177]  [<c1222788>] ? get_super_thawed+0x33/0x147
> [  423.140177]  [<c1246311>] ? iput+0x66/0x320
> [  423.140177]  [<c129eaa8>] sys_quotactl+0x290/0x2fc
> [  423.140177]  [<c18308ec>] syscall_call+0x7/0xb
> 
> As per my investigation, it occurred due to inode_get_rsv_space(inode) lock acquiring if the kernel was built with QUOTA_DEBUG enabled.
> At add_dquot_ref(), the lock did not released if the inode.i_writecount is not *ZERO*, inode is not in I_FREEING|I_WILL_FREE|I_NEW state,
> as well as it need to do quota init.
> 
> if ((inode->i_state & (I_FREEING|I_WILL_FREE|I_NEW)) ||
>      !atomic_read(&inode->i_writecount) ||
>      !dqinit_needed(inode, type)) {
>      spin_unlock(&inode->i_lock);
>      continue;
> }
> 
> #ifdef CONFIG_QUOTA_DEBUG
> 	if (unlikely(inode_get_rsv_space(inode) > 0))
>         	reserved = 1;
> #endif
> 
> In my situation, atomic_read(&inode->i_writecount) returns -2, looks that the related file have two
> deny-writers via mmap at that time.
> 
> To nail down this issue, I refresh formated an ext4 file system, and try to open a file and keep writing to it(so that the atomic_read(&inode->i_writecount) = 1).
> then perform quotacheck and quotaon at the same time, and repeat this process for 4 times, the kernel hang for 3 times, 1 time its ok.
> 
> # python -c "f=open('/ext4/test', 'w'); [(f.seek(x) or f.write(str(x))) for x in range(1, 1000000000, 99)]; f.close()"
> # quotacheck -cvumg /ext4
> quotacheck: Your kernel probably supports journaled quota but you are not using it. Consider switching to journaled quota to avoid running quotacheck after an unclean shutdown.
> quotacheck: Scanning /dev/sda6 [/ext4] done
> quotacheck: Cannot stat old user quota file /ext4/aquota.user: No such file or directory. Usage will not be subtracted.
> quotacheck: Cannot stat old group quota file /ext4/aquota.group: No such file or directory. Usage will not be subtracted.
> quotacheck: Cannot stat old user quota file /ext4/aquota.user: No such file or directory. Usage will not be subtracted.
> quotacheck: Cannot stat old group quota file /ext4/aquota.group: No such file or directory. Usage will not be subtracted.
> quotacheck: Checked 2 directories and 1 files
> quotacheck: Old file not found.
> quotacheck: Old file not found.
> # quotaon /ext4			/* kernel was hang. */
> 
> I also verified that this issue can be reproduced against the latest kernel commit(Mon Apr 23 19:52:00 2012 95f714727436836bb46236ce2bcd8ee8f9274aed).
> 
> Below is a small patch, it looks a bit ugly, but works for me.
  Thanks for the detailed analysis and the patch! You are correct that it
is wrong to call inode_get_rsv_space() with i_lock held. However we cannot
just drop it at that place of add_dquot_ref() because than inode could be
removed from memory before we call __iget(). The right fix is to move
inode_get_rsv_space() to a later place in the function where i_lock is no
longer held. Attached patch should fix the problem.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR

View attachment "0001-quota-Fix-double-lock-in-add_dquot_ref-with-CONFIG_Q.patch" of type "text/x-patch" (1292 bytes)

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